magento plugins

I am getting my first magento webshop. At the moment I am trying to figure out what plugins are the best out there, paid and unpaid.

I always value the opinions of fellow blackhatters very much, so what are your favourite plugins?

One more question, I will have a lot of different products in the shop (duh), are there systems that all big sites use that are very good? Or can I just start a system that makes sense to me... This is something that will really suck if I make mistakes in...

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